Contactless smartcards, such as proximity or vicinity cards, communicate with a card reader by load modulation. The reader receives the load modulation signal as (amplitude) AM modulation of the carrier frequency, typically 13.56 MHz. The amplitude of the antenna voltage varies depending on the load that the card present to the reader antenna. At close range, when the coupling factor is high between the smartcard and the antenna, the antenna voltage can be fairly low, typically 5-10 volts peak to peak (Vpp). At far range, where the coupling factor is low, the antenna voltage can be high, up to 60 Vpp.
Typical reader integrated circuits (ICs) often have the antenna signal routed directly to the receiver without any signal processing except attenuation. These receivers are typically limited to a signal level of less than 3 Vpp. Since the antenna voltage can be as high as 60 Vpp, it can be necessary to attenuate the signal by as much as 20×. This reduces the load modulation at close range to the point where the receiver fails to decode the signal. If the load modulation from the card results in a change in amplitude of 1%, then the amplitude at close range could be as low as 50 mV or 2.5 mVpp after 20×attenuation. The typical minimum detectable signal on the most common reader ICs is 5 mVpp. Thus, typical readers can fail to detect the modulation. Typical readers use a simple attenuator to bring the signal below the maximum amplitude or supply voltage of the IC. Some more advanced receiver designs filter out the carrier from the received signal, but this requires very close tolerances on the components. Improvements in reading load modulated signals in card readers are desired.